Common faults of distribution boxes

reason

1.1 Faults caused by the impact of environmental temperature on low-voltage electrical appliances

The low-voltage electrical appliances in the distribution box are composed of fuses, AC contactors, residual current operated protectors, capacitors, and measuring meters. The upper limit of the ambient air temperature shall not exceed 40 ℃; The average temperature of the surrounding air within 24 hours shall not exceed 35 ℃; The lower limit of the ambient air temperature shall not be less than -5 ℃ or -25 ℃.

The distribution box for rural power grid renovation operates outdoors. It not only receives direct sunlight to generate high temperatures, but also generates heat during operation. Therefore, during the hot summer season, the temperature inside the box will reach over 60 ℃, which greatly exceeds the environmental temperature specified by these appliances.

1.2 Faults caused by product quality

In the transformation of rural power grid, due to the large number of distribution boxes required and the short construction period, the distribution box factory needed a large and urgent supply time for low-voltage electrical appliances, resulting in lax requirements for product quality, which caused some products to malfunction shortly after being put into operation. Some models of AC contactors cannot operate soon after the distribution box is put into operation due to the burning of the contactor closing coil.

1.3 Faults caused by improper selection of electrical appliances in the distribution box

Due to the inappropriate selection of AC contactor capacity during manufacturing, AC contactors of the same capacity were installed for different outgoing circuits, and the imbalance of three-phase load was not taken into account. As a result, the current level of some outgoing contactors could not be increased by one current level based on the normal model selection.

improve

2.1 For distribution boxes with a distribution transformer capacity of 100kV · A or above, consideration should be given to installing temperature control relays (JU-3 or JU-4 ultra small temperature relays) and axial fans at the side wall of the heat dissipation window inside the box. They should be installed on the left side of the box above the control electrical board, so that when the temperature inside the box reaches a certain value (such as 40 ℃), the exhaust fan can be automatically activated to forcibly exhaust heat to dissipate heat from the box.

2.2 Use protective circuits to prevent external circuit faults in the power supply of the distribution box. Choose a smaller intelligent phase loss protector, such as the DA88CM-II motor phase loss protection module (Shanghai product) installed in the distribution box to prevent the motor from burning out due to low-voltage phase loss operation.

2.3 Improve the wiring method of the low-voltage capacitor bank in the original distribution box, changing its installation position from the pile head on the AC contactor to between the low-voltage incoming line of the distribution box and the metering meter. To prevent inaccurate measurement of the measuring device due to phase failure or capacitor damage in the capacitor circuit during operation. In addition, the selection model of the capacitor should be BSMJ series products to ensure reliable component quality and safe operation.

2.4 If a new column mounted distribution stand is added, a 2mm thick stainless steel plate can be selected when making the distribution box shell, and the size of the distribution box should be appropriately enlarged proportionally (based on the JP4-100/3W type used in the agricultural reform project, the width of the original box should be increased by approximately 100mm, that is, from the original 680mm to 780mm. The overall size of the improved distribution box is 1300mm × 780mm × 500mm), in order to increase the electrical safety distance between each branch outlet and between the outlet and the box shell, which is conducive to the operation and maintenance of agricultural electricians and the replacement of melting parts, while also allowing for heat dissipation.

2.5 Choose energy-saving AC contactor (similar to CJ20SI type) products, and pay attention to the connection between the coil voltage of the AC contactor and the corresponding terminal of the selected residual current action protector. Pay attention to the correct load matching. When selecting AC contactors, products with insulation level A or above should be selected, and the rated current of the main circuit contacts must be greater than or equal to the load current of the controlled circuit. The rated voltage of the electromagnetic coil of the contactor is 380V or 220V.

2.6 Selection of residual current operated protectors. It is necessary to choose products that comply with the standard GB 6829 "General Requirements for Residual Current Operated Protectors" and have been certified as qualified by the China Electrical Products Certification Committee. A delay type protector similar to the LJM (J) series, which is energy-saving and low sensitivity, can be selected. The method of protection device should comply with the national standard GB13955-2005 "Installation and Operation of Residual Current Operated Protection Devices". The breaking time of the leakage protector should not exceed 0 seconds when the leakage current is the rated leakage current.

2.7 Low voltage cables should be selected for the incoming and outgoing lines of the distribution box, and the selection of cables should comply with technical requirements. For example, the incoming line of the distribution box for 30kVA and 50kVA transformers uses VV22-35 × 4 cables, use VLV22-35 of the same specification for branching out × 4 cables; The incoming lines of the distribution boxes for 80kVA and 100kVA transformers use VV22-50 respectively × 4. VV22-70 × Use VLV22-50 for 4 cables and branch outlets respectively × 4. VLV22-70 × 4 cables, which are crimped to the copper aluminum wiring nose and then connected to the wiring pile head in the distribution box with bolts.

2.8 Selection of fuses (RT, NT type). The rated current of the total overcurrent protection fuse on the low-voltage side of the distribution transformer should be greater than the rated current on the low-voltage side of the distribution transformer, usually 1.5 times the rated current. The rated current of the melt should be determined based on the allowable overload multiple of the transformer and the characteristics of the fuse. The rated current of the melt for the overcurrent protection fuse of the outgoing circuit should not exceed the rated current of the total overcurrent protection fuse. The rated current of the melt should be selected based on the normal maximum load current of the circuit and should avoid normal peak current.

2.9 In order to analyze the reactive power of rural low-voltage power grid, a DTS (X) series active and reactive two in one multifunctional energy meter (installed on the metering board side) is installed inside the box to replace the original three single-phase energy meters (DD862 series meters), in order to facilitate online monitoring of load operation.